Gibson's publicist Alan Nierob said Monday that Oksana Grigorieva gave birth to a baby girl named Lucia on Friday at a hospital in Los Angeles.
No further details were available.
Gibson has seven children with his ex-wife, Robyn. She cited irreconcilable differences in her April divorce petition to end their marriage of 28 years.
The 53-year-old "Braveheart" star met Grigorieva, a 39-year-old Russian musician, while filming "Edge of Reason."